1. Field of the Invention
The present invention relates in general to electron guns for color picture tubes and, more particularly, to a structural improvement of in-line electron guns for color picture tube for improving the resolution of color picture tube.
2. Description of the Prior Art
In a typical in-line color picture tube having in-line guns or an arrangement of three electron guns in a horizontal line, a deflection yoke having deflection magnetic fields are placed around the neck of the color picture tube. With the deflection magnetic fields of the deflection yoke, there are generated astigmatism and coma aberration of electron beams in the color picture tube and this makes the focus characteristics of a phosphor screen of the color picture tube more deteriorated in comparison with another type of color picture tube.
FIG. 1 shows a structure of a typical in-line color picture tube. As shown in this drawing, the in-line color picture tube 3 comprises a glass panel 1 and a funnel 2, which panel and funnel are integrally formed with each other. Provided in back of the panel 1 and spaced apart from the panel 1 at a given interval is a phosphor screen 9. The phosphor screen 9 is applied with three color phosphors, that is, red, green and blue color phosphors, in the form of vertical stripes on its surface. The three color phosphors convert the absorbed kinetic energy of the electron beams 4 (4R, 4G, 4B) into emitted beam spots. A color selection electrode or a shadow mask 6 is mounted in back of the phosphor screen 9 and spaced apart from the screen 9 at a given interval. The shadow mask 6 is a thin, perforated mask for selection of colors of the three electron beams 4R, 4G and 4B. The holes in the mask 6 are positioned to ensure that the electron beams strike the color phosphor strips of the screen 9. In the above in-line color picture tube, three electron guns 5 for emitting the three electron beams 4R, 4G and 4B are axially arranged in a horizontal line in the neck 7, thus to form the so-called in-line electron guns. In FIG. 1, the axial direction of the color picture tube is Z--Z direction. Mounted around the neck 7 of the tube 3 is a deflection yoke 8 that deflects the three electron beams 4R, 4G and 4B of the in-line electron guns 5.
The inside of the above in-line color picture tube shows a degree of vacuum typically ranged from 10.sup.-6 Torr to 10.sup.-7 Torr.
Representative example of such an in-line electron guns 5 is shown in an enlarged view of FIG. 2. As shown in this drawing, the in-line guns 5 include three electron beam sources or three cathodes 10 that emit their respective electron beams or R, G and B beams and are placed in the horizontal line. The three axes of the cathodes 10 are aligned with centers of their respective openings of two grids or first and second grids 11 and 12. The in-line guns 5 also include main lenses comprising third to sixth grids 13, 14, 15 and 16. The cathodes 10 and the first to sixth grids 11 to 16 are arranged in the axial direction Z--Z of the tube 3 and spaced out at predetermined intervals. The cathodes 10 as well as the first to sixth grids 11 to 16 are fixedly received in a pair of insulating bead glasses 17 of the rod type. The in-line guns further include a shielding cup or a shielding electrode 13 in front of the sixth grid 16. This shielding electrode 18 shields and weakens the leaked magnetic fields of the deflection yoke 8. A heater (not shown) is received in each cathode 10.
Each of the first to sixth grids 11 to 16 is provided with three openings for passing the three electron beams 4R, 4G and 4B of the cathodes 10 therethrough respectively. The three openings of each grid of the guns 5 are arranged in the horizontal line or in the horizontal direction X--X which is perpendicular to the electron beam passing direction. This beam passing direction is equal to the axial direction Z--Z of the color picture tube. The three openings of each grid is also arranged in the same plane.
In the above in-line guns 5, the first and second grids 11 and 12 are plate type electrodes. However, the sixth grid 16 and the fifth grid 15 facing each other are noncircular cylinder electrodes respectively as shown in FIGS. 3 and 4. The sixth grid 16 is otherwise stated as an anode and, hereinbelow, referred to "the second accelerating/focusing electrode" while the fifth grid 15 is otherwise stated as a focus electrode and, hereinbelow, referred to "the first accelerating/focusing electrode".
FIG. 3 is a partially broken perspective view showing representative examples of the first and second accelerating/focusing electrodes. In these drawings, the first and second accelerating/focusing electrodes are designated by the numerals 15 and 16 respectively. The electron beams 4 (4R, 4G and 4B) radiated from the cathodes 10 pass through the first and second accelerating/focusing electrodes 15 and 16 so as to be focused on the phosphor screen 9. Such focusing of the electron beams on the phosphor screen 9 will be referred to FIG. 1.
FIG. 4 is a front view of an accelerating/focusing electrode section of the in-line guns 5 when the tube 3 is cross sectioned at the neck 7. As shown in FIG. 4, each diameter of the circular openings 15a, 15b, 15c, 16a, 16b, 16c of the first and second accelerating/focusing electrodes 15 and 16 is generally ranged from 5.5 mm to 5.9 mm. In each electrode 15, 16, the regular intervals or bridge width l.sub.1 between the openings 15a, 15b, 15c, 16a, 16b, 16c are ranged from 0.8 mm to 1.2 mm. Meanwhile, the interval l.sub.2 between an outside opening of a electrode 15, 16 and the side end of the electrode 15, 16 is ranged from 1.0 mm to 1.4 mm. The circular openings 15a, 15b and 15c of the first accelerating/focusing electrode 15 are offset from the circular openings 16a, 16b and 16c of the second accelerating/focusing electrode 16 respectively by a predetermined distance. It is preferred to let the two groups of openings be offset from each other by a distance ranged from about 0.1 mm to about 1.2 mm. The offset distance of the beam passing openings is affected by both color picture tube size and applied voltage.
In the above in-line color picture tube 3 having three electron guns, the holes in the shadow mask 6 are positioned to ensure that the three beams of the cathodes 10 strike the screen 9. The kinetic energy of each electron beam striking the screen 9 is converted by the phosphors into emitted spots, thus to develop a color image on the screen 9. Here, the emitted spots or pixels caused by the electron beams 4 are important factors that exert a remarkable influence on the resolution of the color picture tube.
The operation of the above in-line guns 5 will be described in detail hereinbelow.
As shown in FIG. 2, the electron beam sources or the cathodes 10 emit their thermions due to heat of their heaters. The thermions are controlled as to their amounts in the electron beams by the first grid 11 and are, thereafter, accelerated by the second grid 12. In this regard, the first and second grids 11 and 12 can be characterized as a control grid and a screen grid respectively.
The second grid 12 is typically applied with voltage not higher than 1000 volt. The third grid 13 is applied with voltage of about 20-30% of voltage of the second accelerating/focusing electrode 16.
Due to potential difference between the second and third grids 12 and 13, a weak electrostatic lens or a pre-focus lens is formed between the second and third grids 12 and 13. At the pre-focus lens between the second and third grids 12 and 13, the diverging angles of the beams 4 or the inclination angles of the beams with respect to the direction Z--Z after the beams are transmitted through the pre-focus lens are determined. That is, the incident angles of the beams at the main lenses were already determined by the pre-focus lens. Hence, the pre-focus lens is an important factor influencing the focus characteristics of the guns 5.
The pre-focus lens has the collateral function of shielding against possible infiltration of electric field of the third grid 13 into the cathodes 10.
After passing through the pre-focus lens, the electron beams 4 (4R, 4G and 4B) are accelerated while retaining their predetermined diverging angles and, thereafter, received by the main lenses. The electron beams are in turn focused on the phosphor screen 9 by the main lenses, thus to generate emitted spots and to develop the color image on the screen 9.
At this time, the second accelerating/focusing electrode 16 of the main lenses is applied with high voltages ranging from about 22,000 volt to 35,000 volt. The first accelerating/focusing electrode 15 is applied with mid-range voltages of about 20-33% of the high voltages of the second accelerating/focusing electrode 16. There is thus generated a potential difference between the first and second accelerating/focusing electrodes 15 and 16 so that the main lenses are formed between the two electrodes 15 and 16. The main lenses affect the focus characteristics of the electron beams 4 (4R, 4G and 4B).
The circular openings 15a to 15c and the circular openings 16a to 16c facing each other are spaced out at an interval of 0.8 mm-1.2 mm. In addition, the circular openings 15a to 15c are offset from the circular openings 16a to 16c respectively by the distance of about 0.1 mm-1.2 mm as described above. In this regard, the main lenses for the side electron beams are axially asymmetrical in the Z--Z direction. The side electron beams are thus converged into the center electron beam so that the three electron beams 4 (4R, 4G and 4B) are converged into a single point.
Such convergence of the three electron beams is a so-called static convergence (STC) of the electron guns 5.
In the above in-line guns 5, each of the main lenses has a small aperture of about 5.5 mm-5.9 mm. The main lenses should be thus affected by spherical aberration.
In an effort to reduce the bad effect of the spherical aberration, a multi-stage beam focusing technique is preferably used.
The electron guns 5 shown in FIG. 2 are guns of the multi-stage focusing type. In the above guns 5, the second grid 12 is electrically connected to the fourth grid 14 while the third grid 13 is electrically connected to the first accelerating/focusing electrode 15.
In the above in-line electron guns 5 for color picture tube 3, one of causes of deterioration of the resolution of the color picture tube is known as haze because of cloudiness formed about the pixels of the beams. When there is haze in the color picture tube, the pixels are not clear but faded. Such haze, which is noted to be caused by both the spherical aberration and the astigmatism, not only deteriorates the sharpness of the pixels of the electron beams but also enlarges the beams spots, thus to deteriorating the resolution of color picture tube.
As will be noted by the equations described later herein, the bad effect of the spherical aberration is in inverse proportion to third power of the aperture size R of the main lenses. The aperture R of the main lenses is in proportion to both the diameters of the first and second accelerating/focusing electrodes 15 and 16.
That is, the focus strength of the main lenses is in reverse proportion to the diameters of the beam passing circular openings 15a to 15c and 16a to 16c of the first and second accelerating/focusing electrodes 15 and 16. Thus, the above in-line guns 5 has a problem in that the proper focusing voltages for the electron beam spots or for the pixels are not same.
When representing in equations, both the axis phase derivative of second order .phi."(z) and the spherical aberration component "c" of potential will be represented by the following equations respectively. EQU .phi."(z).varies.(2/.pi.s).multidot.(V.sub.2 -V.sub.1).multidot.1/R EQU and EQU c.varies.M/16R.sup.3
wherein
V.sub.1 is a voltage of the first accelerating/focusing electrode 15; PA1 V.sub.2 is a voltage of the second accelerating/focusing electrode 16; PA1 s is a distance between the first and second accelerating/focusing electrodes 15 and 16; PA1 M is magnification of the main lens; and PA1 R is aperture of the main lens. PA1 Dsa is a magnified component of the electron beam magnified by the spherical aberration component; and PA1 Dsc is a magnified component of the electron beam magnified by space charge effect. This Dsc will be represented by the following equation: EQU Dsc=f(r.sub.sc /r.sub.i).varies.(i.sup.1/2 /V.sup.3/4).multidot.(Z/r.sub.i) PA1 i is beam current; PA1 V is a high voltage; and PA1 r.sub.sc /r.sub.i is beam spread.
When the aperture of the main lenses is increased, both the focus strength and the spherical aberration component of the main lenses will be thus decreased as represented in the following equations. EQU Focus strength.varies.1/.DELTA.R EQU and EQU Spherical aberration component.varies.1/(.DELTA.R).sup.3
When the aperture R of the main lens is increased so as to overcome the above problem caused by the astigmatism, the pixel size or the resulting beam spot size on the screen 9 will be thus reduced in accordance with the following equation and the resolution of the color picture tube will be thus improved.
When letting the resulting beam spot size on the screen 9 be Ds, the spot size Ds will be represented by the following equation. ##EQU1## wherein Dx is a magnified component of a cross-over point dx magnified by the magnification of the main lenses M, otherwise stated, Dx.varies.M.multidot.dx;
wherein
However in the above in-line color picture tube, the beam passing circular openings 15a, 15b and 15c of the first accelerating/focusing electrode 15 are arranged in a horizontal line of the X--X direction and in the same plane as shown in FIGS. 3 and 4. In the same manner, the beam passing circular openings 16a, 16b and 16c of the second accelerating/focusing electrode 16 are arranged in a horizontal direction of the X--X direction and in the same plane. Therefore, the diameter of each of the openings 15a, 15b, 15c, 16a, 16b and 16c, the openings forming the main lenses, is inevitably limited to a size not larger than 1/3 of the inner diameter of the neck 7 of the color picture tube 3.
In FIG. 4, the diameter of each of the beam passing openings 15a to 15c and 16a to 16c of the first and second accelerating/focusing electrodes 15 and 16 is designated by the alphabet D. The beam separation or the distance between the centers of the beam passing openings 15a to 15c and 16a to 16c is designated by the alphabet S. The minimum gap between each of the electrodes 15 and 16 and the inner surface of the neck 7 is designated by the alphabet g. This minimum gap g is the minimum gap that electrically insulates each of the electrodes 15 and 16 from the inner surface of the neck 7. The beam passing openings 15a to 15c, 16a to 16c are spaced out at regular intervals l.sub.1 while the side openings 15a and 15b, 16a and 16b are spaced apart from the opposed side ends of the electrode 15, 16 by the distance l.sub.2. In the above guns 5, the interval or the bridge width l.sub.1 should be about 1.0 mm. The bridge width l.sub.1 is the minimum width allowing the mechanical machining of electrode.
Therefore, D.ltoreq.S-1 (mm). When letting the inner diameter of the neck 7 be L, the inner diameter L will be represented by the following equation. EQU D+2(S+g+1).ltoreq.L
Practically, the desired electric insulation between each electrode 15, 16 and the neck 7 is not achieved when the gap g is less than 1.0 mm. In this regard, the diameter D of each beam passing opening should be not larger than (L/3)-2 mm. That is, D.ltoreq.(L/3)-2 mm.
The diameter D of each beam passing opening of the electrodes 15 and 16 should be thus limited to the size not larger than 1/3 of the inner diameter L of the neck 7.
Therefore, when enlarging the diameter D of each beam passing opening of the electrodes 15 and 16 of the above described in-line guns 5, either the beam separation S should be increased or the inner diameter L of the neck 7 should be enlarged.
In the typical color picture tube, the electric power to be used for deflecting operation of the deflection yoke 8 should be increased regardless of shapes of the electrodes 15 and 16. Furthermore, such lengthened beam separation S is attended with deterioration of beam convergence characteristics of the main lenses, thus to deteriorate the resolution of the color picture tube.
In order to combat the above problem of deterioration of resolution of the color picture tube, Korean patent Publication No. 89-3825 and Japanese Patent Laid-open Publication No. Sho. 59-215640 disclose improved in-line electron guns for color picture tubes. In each of the above Korean and Japanese in-line guns, effective lens aperture is enlarged by provision of a large aperture of envelope electrode, which envelope electrode also includes therein a backward elliptic plate. In addition, part of the side beam passing openings is removed, thus to somewhat remove the astigmatism of the side beams.
However, the backward plate of the above guns is a flat plate and this practically makes the bridge width between the beam passing openings be not larger than 0.5 mm. Therefore, the desired strength of the electrodes can not be achieved. With the weak strength of the electrodes, the electrodes are apt to be deformed by tools which will be inserted in the beam passing openings when assembling the electrodes into the in-line guns. In addition, the backward plate may be deformed at its bridge portion by welding pressure when welding the bead glass for retaining the gaps between all the electrodes of the guns. In this regard, the above guns disclosed in either the Korean patent or the Japanese patent still deteriorate the convergence characteristics of electron beams or increase the astigmatism, thus to fail in achieving the desired resolution of color picture tube.
As described above, the typical electron guns for color picture tube have a problem that the inner diameter of the neck of the color picture tube can not be enlarged. Another problem of the typical in-line guns is resided in that there should be limit in enlarging both the vertical width and the horizontal width of the beam passing openings due to the structural limit of the beam passing openings. The bridge width between the beam passing openings should be thus narrowed.
Such narrow bridge width of the electrodes of the typical in-line guns weakens the mechanical strength of the electron guns and, as a result, causes a lot of bad products in producing the guns. Furthermore, it is very difficult to treat and handle such guns so that productivity of the guns is adversely impacted.